Gas turbine nozzle and bucket shroud structure



Aug. 12, 1952 A HOWARD 2,606,741

GAS TURBINE NOZZLE AND BUCKET SHROUD STRUCTURE Original Filed June 11, 1947 2 Pi .l. 2 $36 @1 2 C) v G /5 I 37 Q His Attorney.

Patented Aug. 12, 1952 2,606,741 my OFFICE,

GAS TURBINE NOZZLE A D BUCKET? snaouo STRUCTURE;

Alan Howard, Schenectady; N. Y., assignor to 7' General Electric Company, a corporation of New York l Original application June 11, 1947, Serial No.

754,002. Divided and this application July 28, 1949, Serial No. 107,306

- Thisinvention relates to gas'turbines and more particularly to the nozzle and bucketshroud structure thereof. 1 This is a division of application Serial Number 754,002 filed June 11, 1947 in the-names of Alan Howard, Chester S. Rice and'Bruce 0. Buckland and assigned .to the same assignee 'as the present application. a H

vIn the design of gas turbinepowerplants, a major consideration is theprovision of-arr'angements for permitting free diilerential thermal expansion between relative parts without'producingxundes'ira'ble'deformation of the rotor or material alteration of, the clearances between the rotor and ,va'riousistationary parts. Because of the "extremelybigh temperatures tofiwhich oer-i taint parts must necessarily be subjected, it is necessaryyto use. special high "temperature resisting materials, such as various stainless steels. These generally have a coefiicient of:-expansion .in the neighborhood of twice that of ordinary mild steel, with the result that utilizatio'nlof such materials magnifies: the problems resulting from differentialzthermal expansion; Further? more, in a powerplant designed for applications where the load must be frequently 'and materially altered,- asrfor instance in marine installations and: locomotives,- it' is found that differential thermal expansion problems 'are intensified still furthe'rby difierences in the size and mass. of

various related parts and difierences' in the 'resistance of the heat flow paths to themu'from the'source of .heat, with accompanying differences in theprateof change' of dimension" when temperatures changerapidly. l.

An object is to provide animp'rovednozzle structurerfor a gas turbinespowerplant, capable of 'operation at high; rapidly changing temperaturesuwithout transfer of excessive'heat or dis torting forces to associated-parts.

V A further object is to provide an improved stationary shroud arrangement for turbine 'buck ets of th'e'open or shroudle'ss' type.

1 Other objects and 'advantage's will be apparent from the following description taken in connection with theaccompanying drawings'in which Fig. "1 is an enlarged sectional viewof the turbine nozzle structure, bucket wheels,- interstage diaphragm, and outer turbineca'sing;' and Fig. 2 isa sectional view broken awayalong the 'ir-regular plane 222 in Fig. l.

Referring now to Fig. 1, aga's'turbine 5 includes a rotor '6, having 'a firetstage turbine bucket wheely indicated generally 'at 1, "which in turn has a, hub portion which may be connected by any one'of several means. to a 100mpressor (not shown). The hub portion ofwhe'el -1 has a first radially extending web portion '8,

to which is welded a eircumfereritial' 'rim;por-

tion 9; which in turn carries-acircumferential '4 Claims. (01. ass-39.15)

row of buckets 10. These buckets may be secured to rim -9;by any suitable means, such as dove-tails, welding etc. It will be noted that the buckets H} are of the shroudless or openended type. This construction is what has come to be known asa composite bucketwheel, which is more fully described and claimed in an-ap plication, Serial-No. 498,643, filed August 14, 1943, in, the name of Alan Howard, now Patent 2,432,315, issued December 9, 1947. This construction 'is'characterized by the fact that the central web portion 8 may be readily forged of ordinary mild steel, while therim portion 9 is fabricated separately of a-high temperature're sistant materiaL'such as an austenitic steel. A- second-stage turbine bueketewheel, indicated generally at Hand secured to wheel 1 by means of acircumferential row of bolts [2a projecting through cooperating flanges formedintegral with, thejbucket-wheels, is also of the composite type and carries a circumferential row of shroudless second-stage buckets l2.

Hot motive fluid received from eombustor discharge sections 13 is delivered to'the buckets ID of" the first stage'wheel I by means of a nozzle ring assembly, supported at its innerperiphery by means of a heavy flange member l4 secured to the end-of an axially extending cylinder [5 which in turn'forms an integral part of the main turbine frame structure. Secured to fiange M by a plurality of threaded fastenings 15a is an annular nozzle support plate [6. Secured by bolts I] to the outer circumference of nozzle support plate IB-is a convoluted ring indicated generally at, I8 whichhas an inner-portion 19 extending axially to the left, an intermediate radially extending :portion to which combustor discharge support lugs 28 are welded, and an outer wall 2| extending axially to the right 'toward the turbine bucket-wheel and forming the inner surface of the nozzle flow path. The outer flow path of the nozzleringassembly is formed by asecond convoluted member 22, including an inner-wall 2 3 welded to, or formedintegral with, anouter axially extending wall 24. It will be apparent 'thatthe intermediate *portion of the convoluted rings [8 and 22 define an annular opening into which freely projects the *discharge portions of the combustor transitionsections 13. It should also --be noted that the ring portions l9, Hare comparatively wide in an axial' direcwtionandare sufficiently thin to have appreciable flexibility in a radial direction; Because they .are thin, they present a minimum flow path for the conduction of heat from the hot nozzle assembly to the related comparatively cooler supporting structures.

The bolts, ll also serve :tosupport a packing ,ringassemblyincluding an annularplate 25 having at its inner periphery-several concentric packing rings 26 projecting axially and forming close clearances with the adjacent rim portion 3 of bucket-wheel I. At its outer periphery, plate 25 carries additional packing rings 21. The outer periphery of plate 25 also forms a radially slidable joint with an annular flange 28 at the end of wall 2|. As noted above, the convoluted ring [8 has appreciable flexibility in a radial direction so that the nozzle may expand and contract readily relative to the support plate I6 and the sealing ring plate 25. To this end a generous radial clearance space is provided between the periphery of plate 25 and the inner circumferential surface of wall 2!. The area of contact between flange 28 and the periphery of plate 25 is kept to a minimum so as to reduce the'flow of heat by conduction therebetween.

An additional set of sealing rings I611 are secured to the nozzle support plate [6 and cooperate with anaxially projecting boss 80 on the adjacent web portion of bucket-wheel I. The provision of the projecting annular boss 811 has the advantage that in the event of accidental rubbing between packing rings 16a and the bucket-wheel, the boss 8a may be scored without seriously weakening the web of the turbine wheel. As will be understood by those skilled in the art, any small-scratch or groove in the surface of a highly stressed rotor constitutes a stress raiser which may rapidly lead to failure of the rotor member. A boss such as that shown at 80!. effectively prevents weakening of the bucket-wheel web 8 by any grooves cut by the sealing rings l6a.' This packin'gring arrangement is more-fully disclosed in United States Patent 2,564,503, issued August 14, 1951, in the name of Chester S. Rice and assigned to the same assignee as the present application.

Secured in the inner and outer nozzle ring walls 2|, 23 are a plurality of radially extending circumferentially spaced nozzle blades 29. These blades project through suitably shaped openings in both the outer wall 23 and the inner wall 21, but are welded only at the outside of the'wall 23, as indicated at 30. The radially inner ends of blade 29 are free to slide in and out of the slots in wall 2| through which they project. Thus the inner and outer nozzle wallsmay freely expand and contract relative to each other with out imposing buckling stresses on the comparatively thin blades 29. Conversely, the comparatively thin blades 29, which heat morerapidly than the walls 2|, 23 when suddenly subjected to hot motive fluid, may expand and contract freely. a

'At its right-hand edge, the outer nozzle support ring 24 is provided with a radially extend ing flange secured by bolts 3! to the inner periphery of a main support ring 32, Also secured by bolts 3l is a shroud member 33. This is not a solid ring but is made up of a plurality of segments, as may be seen better in Fig. 2. Each segment is held by one bolt 3|, and is so dimensioned as to form an appreciable clearancespace 33a with adjacent segments when cold. At operating temperatures, this'clearance space 33a substantially closes up so that together the blocks 33 form a continuous annular shroud for the open-ended or shroudless buckets I0. -At the inlet side, each shroud block 33 is provided with a groove shown at 34 in Fig. 1 and adapted to pivotally receive one edge of a bridge member 35. Member 35 is not a continuous annular ring 'but is also made up of a plurality of segments,

each corresponding in circumferential length to that of the segmental shroud blocks 33. At their inlet edges. bridge segments 35 are provided with a second portion adapted to be pivotally received in a groove 36 formed in the adjacent end face of the outer nozzle wall 23. With this arrangement, the nozzle wall 23 may freely expand and contract radially relative to the comparatively cooler shroud segments 33, during which movement the bridge members 35 rotate slightly while preserving a substantially continuous wall for the flow of fluid from the nozzles to the turbine buckets.

To further minimize the flow of heat from the hot nozzle ring assembly to the comparatively cooler main frame rings 32 and 32a, the latter of which is secured by bolts 36 to ring 32, suitable heat insulating lagging is provided. This may be in the form of an annular box 31 fabricated of sheet metal and containing suitable insulating material such as asbestos or magnesia. Box 31 may be secured to the main frame ring 32 by means of bolts 38.

In addition to the bolts 36, the main frame rings 32, 32a are clamped together by an inner row of bolts 36a. Intermediate these two circumferential rows of bolts are a plurality of spaced cooling air holes 39. It may be noted that main frame plate 32a is cut away or scalloped as indicated at 32b in Fig. 2 so as to provide radially extending projections through which the bolts 36 pass. On the other hand, the main frame ring 32 has a smooth circular outer circumference. The scallops 32b are provided merely for weight saving.

The main frame ring 32a is supported in fixed relation to the inner frame cylinder I5 by an axially extending member I5d welded to a radially extending plate l5b. Plate l5b has a plurality of circumferentially spaced openings I50,

through which the respective combustor transition sections I3 project.

The entire design of this nozzle ring and shroud assembly is carefully arranged to reduce the transfer of heat from the hot parts to the cool frame members 32, 32a, etc. It will be ob v served that the convoluted ring 22 presents a comparatively long path for conduction of heat from the nozzle ring to the support member 32.

and this flow path is of high resistance because of the small cross-section area of wall 24. The heat insulating box 3! also prevents radiation from the hot nozzle ring assembly to the cool rings 32, 3201.. Care is taken throughout that metal-to-metal contacts between adjacent parts are kept to a minimum so as to limit the conduction of heat. With the arrangement described, the nozzle blades and walls heat very rapidly when. subjected to hot motive fluid and are maintained continuously at a high temperature, being permitted to freely expand and contract relative to the comparatively cooler adjacent supporting members as the temperature of the motive fluid varies.

When shroudless turbine-buckets, such as those shown at ID, are used, it is essential to the efllciency of the turbine that the radial clearance space between the bucket tips and the adjacent stationary shroud 33 be maintained at a preselected small value, which however may not decrease below a certain minimum value if mechanical interference between buckets and shroud is to be prevented. This problem of maintaining proper bucket-to-shroud clearances becomes almost impossible to solve if both the bucket-wheel and the shroud are caused to change-dimensionsrapidlwbut at differing-mates, as the temperature ofthe motive' fluid varies. The: present a arrangement facilitates the maintenance of; proper "clearances by the provisionof f-the inairi framering 32which is carefullycooled "and arranged" so that ---its -temperature doesnot vary sufli'c'iently tomaterially alter-its radial dimensions. l -The shroud--se'grnents-- 33,=--which:; are in direct contact with"-the--motive fluidg are-per- "i6 i The==inner wall .of 'the motiveflflow path lis -'-formed byanannular wall: 53,: which at its 'left- 1; hand --edge is provided with an inwardly extending flange": 54 -andat its I right-hand edge is :pro- 5 -vided- 'with a second inwardly. extending portion EBJ SeGured to .wall portion 55 is an axially exi tending wall fad which is -thin enough in section to be somewhat flexible and to present a high: resistance heat flow path. Secured -to wall: 56

-'mitted to -freely expand and contract circum-L ferentially. lay-reason of the w clearance spaces 33a as shownii-i Fig. 2i-w-ith the :result: that the inner diameter of 'the shroud ring :formed by the .segments 33 remains substantially-- constant. 1 Thus -itrisesto operating temperature, a safe'; minimum clearance space between bucket -'.tips and "'shroudwill. result. i

As shown in Fig. 1, an interstagecasing 40 is made in two halves bolted together-on a: vertical: plane through the axis of the powerplant. This casing is-channel-shaped incross-section} having cylindrical -wall -40a: and radially extending fia'nges' 4011 and "40cadaptedl2to be secured by te a second frame ring-"425 Ring 42 -is-prov-ided with a series of circumferentially' spacedcooling air holes 43 arranged similarlyto theholes 39 in fram'e ring SZV: Though Q1101} :material to an -understanding of the-present"invention,' the cool-w --'-ing-' system ser'vioedbyholes 39t'and 43 is more particularly described application I Serial Number 754 002 filed June 11, 1947-, inthe names "of Alan-Howaid- "Chester S.-Rice and Bruce 0. iBuckland and assigned: td the same assignee as i thelpresentapplication.

.i Projecting radially-inward from-thecasingw'all 40a l are: walls defining an annular cooling air liehamb er 44. -Projecting inwardly asa continu- .wp'OIitiOnfOf ring 4'1 may be' secured by suitable ':.means,:. for instance gufew tack welds, to the pyinnericircumference of wall 40d. "Because'of the a reverse) curvature of ring" and the fact that his r fabricated of comparatively light gagesheet metaLa it will resilientlyseal the space between :the'swall 46a and shroud. segments 33 while permitting relative thermal between. r r

1 It will beobserved that the comparatively thin -fl exi ble :walls :45," 46 form: a convoluted ring quite similar in arrangement. and purposes to the rings 1 48 22 of ,.the-;first=stagei nozzle assembly. 7' To further... limit the transfer 30f: heat from: the hot ginterstageqnozzle assemblyto the cooler/casing V .-40,---an -annular fabricated sheet metal .box '48;

yfilledwith heat insulatingmaterial maykibe rse- ,ctrea by bolts 49 to wall 45. Similar insulation filled spaces are provided by sheet metal rings I fifljlfil andiz, whi'ch maybe secured'inplaceby suitable vthreaded fastenin s or tack-welds.

1 expansion *therean i inwardly extending wall '51 which carries a casing 58 defining an annular cooling air-passage 59- and servingto support a-labyrinth typefpackingememb'er' (it. -As: will be obvious from Fig-C :1, v the 'labyrinth seal memberbn cooperates withitthe tthe -bucket-whe'el can; be so designed that: when:v 1'

outer circumference of the -bolting flanges "through which project the turbine coupling bolts 1 2a. As shown in the drawings, packingmem-ber 160 :is intended: to represent awell-known type of upariking, I many various forms 'of which: are

familiar to J those acquainted with i the conven- 'tional"steam:.-turbine art. Secured to walli5l=by screws 6 I is t a sealing. plate member -62 which -=supportstwosetsof packing rings 63,64 similar "in construction and purpose to the ringe -Ilia,

.26. Secured to the outer circumference of mem- -ber:62 isian annular thin-walled reversely curved .m'ember'; 65 having an outer circumferential portion 66. Ring 66 is in slidable engagement with :.:the. sidewall of fiange- 54," and forms a radial clearancespace with the 1 inner surface. of wall thermal-expansion of-the free left-hand edge of .wall 53. WalL 53 and the above-described parts associated with it are'supported from the outer .lflow-path: wa1l545 by means of -theradially extending diaphragm blades 61'. Because V of the a comparatively lower temperature of the motive fluidsin this .interst'age passage, blades 61' may be welded both to the outer wall 46 and the inner wall 53 asshown inl 'ig. lpalternatively, only 1':the-.-in'let portions ofthe' blades may be welded to. thewal1s'. 53; the thin discharge portions ;o'f.-the blades-being left free to expand without buckling. Bolted tothe right-hand side'of casing-158; is an outwardly extending annular sealing plate 68x having. a circumferential portion 1 69 .c'arrying; packing rings 'Hl which form a close clearance with the adjacent rim portion of bucket wheel |l.- 'Ring'B9 is in radially slidable 1 Asalso shown in Fig. 1; mainframering fl supportsa 'shroud-42a'which is similar inarrangeme'nt and function 'to the segmental shroud Bi-associated withj'the first turbine "stage. At

function. to ringti.

*the'left handedge'of shroud 42a; the clearance space withfnozzle' walls 45, '46 isseated byfaf fie'x ible; S-fingVtack-welded totheflinner periphery 0f"fiange40c,' and similar. in construction. and At 'its right-handledge, shroud 42a is sealed to' the adjacent edge ofran exhaust. casing wall 1 Iby a thin flexible annular bridge ring 12, which may be welded to wall H .and projects freelyinto a circumferential groove ,in'the adjacent .face of the shroud blocks..42a,.

While the thinsupporting walls It, 24 for the nozzle ring. assembly and walls 45,56 of the interstage casing have been described as being "flexible in the radial direction, it is desired to [call attention to-the fact that these wallsare not .;perhaps flexible in the; ordinary sense 'of. the

word. As described-above, these walls are of very thin section and considerableaxial length, -so as to present a high resistance path for the flow-of heat by conduction from the hot parts to the comparatively cooler main frame parts. Be-

cause of theirthin cross section, it appears inthe drawings that they would be flexible in'a radial direction. Howeverit must beremembered that each-of these walls is a continuous annular member, which is comparatively rigid in the sense that if one end of the ring member were held fixed, it would strongly resist any force tending to move the other end portion transversely, in

vaplane parallel to the first end portion. In other words, theseannular wall members are rigid in the sense that they tend to maintain their respective end portions exactly coaxial. However,

. when one end of these ring membersis strongly heated, as by conduction from a hot member to whichthe ring is connected, there will be a rather steep temperature gradient established along the axial length of the ring, by reason of the high resistance, of the heat conduction path. Furthermore the opposite end of the ring, for instancethe flanged right-hand end of wall 24, will be maintained cool by its intimate contact with the comparatively cool supporting member to which it is attached. Accordingly, the free end of these thin walled rings tends to expand radially much more than the cooler fixed end of the ring. The ring is flexible in the sense that it readily permits this type of differential thermal expansion without creating excessive stresses in the ring or imposing excessive stresses on associated parts. However, as described above, the ring rigidly maintainsits end portions coaxial. This is important for it is obviously necessary to keep the nozzle ring assemblies exactly coaxial with the respective bucket-wheels, in orderto preserve thethermodynamic eflficiency of the turbine and prevent mechanical interferences.

For cooling and sealing the interstage' casing assembly, cooling air is supplied by means of a conduit 13 to the annular chamber 44 in casing 40, thence through a plurality of circumferentially spaced conduits 74, one of which projects radially inward through each of the hollow nozzle blades 61; discharging into the cooling air passage 59. A plurality of circumferentially spaced passages 15 delivers cooling air from chamber 59 to an intermediate portion of the labyrinth seal 60. It wil be observed that the point at which passage 15 enters the labyrinth seal is closer to the left-hand side of the seal than to the right-hand side. This is because there is an appreciable pressure drop across the nozzle diaphragm blades 61, which means that the pressure 1 in the space adjacent'the web 8 ofthe firststa'ge Wheel is appreciably higher than that obtaining adjacent the web of the second-stage wheel I I. By causing the cooling air conduit I5 to discharge into. the seal closer tojthe highpressure side the flow of cooling air leftward into the space adjacent bucket-wheel 8 can be made substantially equal to the flow which passes wheelv II and is discharged in the manner of the arrow 1! into the motive fluid flow path.

It will be seen from the above that comparatively'high-pressure airadmitted through conduit .13'is"caused to cool the labyrinth seal mem- .-berso that itsv desirably close clearances with a the. rotormay be maintained, this air also servshown) to the-annular space labeled 78 Fig. 1.

From this annular chamber, cooling air flows in the directionindicated by the arrow 19 radially outward along-the web 8 of the'first-stage wheel past the sealing rings [6a, 26, 21 carried by plates [6, 25 and into the motive fluid flow path as indicated by the arrow in Fig. l. 1

While a particular embodiment of the inven- 'tion has been illustrated and described, it will be obvious to those familiar with the art'that various changes and modifications may be made without departing from the invention; and it is intended to cover in the appended claims all such changes and modifications as come within the true spirit and scope of the invention. 3 .WhatIclaim is: r v

1'. In a high temperature turbine having an axial flow bucket-wheel with a circumferential row of buckets, the combination of a main frame including a nozzle support ring coaxial with the bucket-Wheel. and av main frame'ring surrounding the bucket-wheel and radially spaced therefrom, a nozzle ring assembly adapted tosupply motive fluid to the buckets and including. an inner ring member connected to the nozzle support ring by a thin axially extendingportion having appreciable flexibility in a radial-direction,

an outer ring member, and a plurality of circumferentially spaced nozzle blades engaging said outer and inner nozzle ringmembers, means supporting the outer, ring from the main frame ring including a thin annular-member havingone end portionsecured to the inlet portion of the outer nozzle ring and a second end portion secured to the mainframe ring substantially in the plane of adjacent segments'by a small clearance space when .cold whereby diiferential thermal expansion between the segments and the main frame ring-may'take place when heated, said segments defining a circumferential inner surface forming asmall radial clearancewith the tips of the buckets, the discharge edge of said outer nozzle ring and the inlet edge of said shroud segments being providedwith axially spaced parallel'arcuate-grooves, and a plurality of bridge segments having end portions pivotally engaging said respective grooves, the circumferential length of the bridge segments corresponding to that of the shroud segments, whereby differential thermal expansion between the nozzle ring assembly and the shroud segment assembly may freely take place, a'sealin'g plate assembly secured to said nozzle support ring and having an outer circumferential edge tloosely'sealed to the inner nozzle close clearances -with the' ring and a plurality of sealing ring members havmg concentric annular edge portions forming x adjacent face of the bucket-wheel rim, and means for cooling the main frame ring to minimize changes in the radial dimensions thereof during operation.

2. In a high-temperature turbine, a nozzle ring assembly comprising inner and outer axially extending radially spaced concentric annular walls defining a flow path for high-temperature fluid, a plurality of circumferentially spaced nozzle partitions extending radially across the flow path and engaging said annular walls, a main support ring member arranged concentric with the nozzle walls and axially spaced from the plane of the partitions, an annular sealing plate assembly arranged concentrically within and radially spaced from the inner nozzle ring wall, a first annular supporting wall having an axial length at least equal to the axial length of the outer nozzle ring wall and surrounding and radially spaced therefrom, said supporting Wall having a first end portion secured to the outer main frame ring and a second end portion secured to the remote end of the outer nozzle ring wall, said first supporting wall being of comparatively thin cross-section so as to offer high resistance to the flow of heat by conduction from the outer nozzle ring to the main frame ring and to permit free relative thermal expansion between the respective end portions of the support wall, a second annular support wall arranged within and radially spaced from the inner nozzle ring wall and. having a first end portion secured to the outer circumference of said sealing plate assembly substantially at the plane of one end portion of the inner nozzle ring wall, said second support wall being also of thin cross section and having a second end portion secured to the other end of the inner nozzle ring wall, a second sealing plate assembly secured to said first sealing plate assembly and having an outer circumference loosely sealed to the adjacent end of the inner nozzle ring wall whereby hot motive fluid is prevented from circulating into the annular space defined between the second support wall and the inner nozzle ring wall while the latter is free to expand and contract relative to the second sealing plate assembly, and other flexible means bridging the space between the outer main frame ring and the adjacent end of the outer nozzle ring wall whereby differential thermal expansion is freely permitted therebetween while preventing the entrance of hot motive fluid into the space defined between the first support wall and the outer nozzle ring wall.

3. In a high temperature turbine having an axial flow bucket-wheel with a circumferential row of buckets, the combination of a main frame including a nozzle support ring coaxial with the bucket wheel at the inlet side thereof and a main frame ring surrounding the bucket wheel and supported in radially spaced relation to the nozzle support ring, a nozzle ring assembly adapted to supply motive fluid to the buckets and including an inner ring member connected to the nozzle support ring by a thin axially extending portion, an outer ring member, and a plurality of circumferentially spaced nozzle blades engaging said outer and inner nozzle ring members, means supporting the outer ring from the main frame ring including a thin annular member of substantial axial length having one end portion secured to the inlet portion of the outer nozzle ring and a second end portion secured to the main frame ring substantially in the plane of the bucket-wheel, stationary shroud means for the buckets including a circumferential row of separate shroud segments, means 10 securing the respective shroud segments in fixed relation to the main frame ring, each of said segments being separatd from the ends of adjacent segments by a small clearance space when cold whereby differential thermal expansion between the segments and the main frame ring may take place in a circumferential direction when heated, said segments defining a circumferential inner surface forming a small radial clearance with the tips of the buckets, and means for cooling the main frame ring whereby the diameter of the inner circumferential surface defined by the shroud segments does not change appreciably during normal operation.

4. In a high temperature turbine having an axial flow bucket-wheel with a circumferential row of buckets, the combination of amain frame including a nozzle support ring coaxial with the bucket-wheel and a main frame ring surrounding the bucket-wheel and supported in radially spaced relation to the wheel, a nozzle ring assembly adapted to supply motive fluid to the buckets and including an inner ring member connected to the nozzle support ring by a thin axially extending portion, an outer ring member, and a plurality of circumferentially spaced nozzle blades engaging said outer and inner nozzle ring members, means supporting the outer ring from the main frame ring including a thin annular member of substantial axial length with one end portion secured to the inlet portion of the outer nozzle ring and a second end portion secured to the main frame ring substantially in the plane of the bucket-wheel, stationary shroud means for the buckets including a circumferential row of separate shroud segments, means holding the respective shroud segments in fixed relation to the main frame ring, each of said segments being separated from the ends of adjacent segments by a small clearance space when cold whereby difierential thermal expansion between the segments and the main frame ring may take place in a circumferential direction when heated, said segments defining a circumferential inner surface forming a small radial clearance with the tips of the buckets, and means for cooling the main frame ring whereby the diameter of the inner circumferential surface defined by the shroud segments does not change appreciably during normal operation, and a sealing plate assembly secured to the nozzle support ring and having an outer circumferential edge loosely sealed to the inner nozzle ring and a plurality of sealing ring members having concentric annular edge portions forming close clearances with the adjacent face of the bucket wheel rim.

ALAN HOWARD.

REFERENCES CITED The following references are of record in the flle of this patent:

UNITED STATES PATENTS Number Name Date 1,378,464 Junggren May 17, 1921 2,427,244 Warner Sept. 9, 1947 2,432,315 Howard Dec. 9, 1947 2,488,875 Morley Nov. 22, 1949 2,527,445 Pentheny Oct. 24, 1950 2,564,503 Rice Aug. 14, 1951 FOREIGN PATENTS Number Country Date 687,108 Germany Jan. 23, 1940 696,062 Germany Sept. 10, 1940 

